A fuel injection system used for an engine of a motorcycle or the like is typically includes a fuel pump for supplying fuel under pressure, a regulator for keeping the pressure of fuel (fuel pressure) constant, a fuel injection valve from which fuel in an intake passage (or pipe) is injected, the intake passage joined to a combustion chamber of an engine, an electronic control unit (ECU) that is operation control means for the fuel injection valve, and the like. In such a system, the electronic control unit determines an air-fuel ratio at which the most effective combustion condition is achieved, based on information such as an accelerator opening degree, an engine RPM (revolutions per minute), and an intake air amount, and causes fuel to be injected in the amount necessary to achieve such an air-fuel ratio from the fuel injection valve.
Furthermore, another fuel injection system is known as an improved version of the above-described fuel injection system. In such a system, fuel injection valves are provided in the intake passage on the upstream side and downstream side thereof, respectively. Both of these fuel injection valves are connected in series with a fuel pipe joined to a fuel tank. With this configuration, while fuel is constantly injected from the fuel injection valve provided on the downstream side of the intake passage, fuel is also injected from the fuel injection valve provided on the upstream side of the intake passage when an engine load is increased (e.g., Japanese Patent Application Laid-open No. 2004-100633 (JP '633).) It has been known that the fuel injected from the fuel injection valve provided on the upstream side of the intake passage is improved in volumetric efficiency, since heat is taken from intake air when the fuel is vaporized. Accordingly, the fuel injection system with this configuration makes it possible to improve the output of an engine (See, for example, JP '633).
However, in a case where the fuel injection valves are provided on both of the upstream and downstream sides of the intake passage as described above, the distance between the fuel injection valve provided on the upstream side of the intake passage and a combustion chamber is greater than that between the fuel injection valve provided on the downstream side of the intake passage and the combustion chamber. As a result, the fuel injected from the fuel injection valve on the upstream side reaches the inside of the combustion chamber after the fuel injected from the fuel injection valve on the downstream side reaches. For this reason, in order to supply fuel in the whole amount required to the combustion chamber within a period of time in an intake stroke, it is necessary to make the amount of fuel injected from the downstream side larger than that of fuel injected from the upstream side. This brings about a problem that an effect obtained by additionally providing the fuel injection valve on the upstream side of the intake passage is not sufficiently produced.